Required mass flux ${\dot{M}}_{\text{in }}$ (in earth masses per year) against the cloud’s geometric optical depth τ_g (left panel). Both the case where there is no gas (solid line), as well as the one with gas (dashed lines) are given. The latter is plotted for different values of the gas surface density, ∑_gas. In the right panel the required mass flux, ${\dot{M}}_{\text{in }}$ (in earth masses per year), is plotted against the size of the smallest particles, s_min. Both the case where there is no gas (solid line), as well as the one with gas (dashed lines) are given. The latter is plotted for different values of the gas surface density, ∑_gas.